This invention relates to an organic composite coated steel plate that is primarily intended for press forming to make an automotive body plate that has improved corrosion resistance in as-worked state.
In response to the growing need for increasing the corrosion resistance of automotive bodies, the car industry is recently expanding the use of cold rolled steel plates on the bodies of automobiles after they are plated with either zinc or zinc alloys. Included among such "surface-treated steel plates" are plated ones such as those prepared by hot dipping molten zinc or alloyed zinc and those prepared by electroplating zinc or zinc alloys. However, much higher corrosion resistance has been required in curled or hemmed portions of inner plates of car bodies where paint coatings as applied after car body assembly often fail to be deposited in the necessary thickness.
Automotive steel plates that are considered to be suitable for use in such areas were proposed by Japanese laid-Open (kokai) Application Nos. 57(1982)-108292 and 58(1983)-224174 and they are organic composite coated steel plates that are produced by applying a chromate film and a layer of organic high polymer resin on zinc or zinc alloy plated steel plates. These organic composite coated steel plates share the common feature that a paint containing a water-soluble or water-dispersed organic resin and a water-dispersed silica sol is applied on top of a chromated and zinc or zinc alloy plated steel plate so that the latter will develop high corrosion resistance. However, such organic composite coated steel plates have had the following problems:
(1) Since the water-soluble components remain in the formed organic composite coat, the resistance of the coat to the release of chromium is insufficient to prevent chromium from dissolving out of the composite coat during a subsequent chemical conversion treatment, and the released chromium can be a cause of environmental pollution;
(2) During alkali degreasing, the resin layer will separate from the substrate, thereby deteriorating its corrosion resistance;
(3) When the steel plate is used in a corrosive environment, water will enter the resin layer and the soluble components will dissolve to provide a highly alkaline condition, whereby the adhesion between the resin layer and the chromate film will deteriorate.
With a view to solving these problems, a method characterized by using a paint composition that contained not only a hydrophobic silica having its surface substituted by organic matter in the presence of an organic solvent but also an organic resin such as an epoxy rein was proposed in Japanese Laid-Open (kokai) Application No. 63(1988)-22637. This method has the advantage that the compatibility between the silica sol and the organic resin is insured and that, in addition, strong adhesion is achieved in as-painted state. However, the applied coat is not flexible enough to prevent damage from occurring in those areas of the coating layer which have been subjected to a working operation such as pressing and, as result, the corrosion resistance of those areas is unavoidably deteriorated.
This problem of low corrosion resistance in an as-worked state has been addressed by Japanese Laid-Open (kokai) Application No. 62(1987)-289274, which proposes a method characterized by providing a coating layer that contains a urethane base resin and silicon dioxide as main ingredients. The method achieves some improvement in workability but as it turned out, deterioration in the corrosion resistance of the thus coated one was still unavoidable when it was subjected to working under severer conditions. A particular problem occurred when an aqueous resin was used; namely, depending on the combination with water-dispersed silica, the aqueous resin did not have sufficient compatibility with silica to permit the preparation of an effective paint.
Fingerprint-free steel plates for use in home electric appliances have been proposed in Japanese Patent Publication (kokoku) Nos. 61(1986)-36487 and 1(1989)-44387. The methods they teach comprise forming a chromate film on a steel plate that has been electroplated with a zinc alloy, applying an aqueous solution comprising a carboxylated polyethylenic resin dispersion and a colloidal silica, and drying the applied solution to form a composite coating. The steel plates produced by these methods are evaluated by a salt spray test but the test conditions differ so greatly from the environment under which the steel plates are to be used on automotive bodies that the composite coating formed by the methods under consideration have failed to exhibit satisfactory performance in a corrosion test conducted under the conditions specified by the present invention. It should also be noted that the steel plates produced by those methods did not bring about good results when tested in accordance with the present invention.
A further problem with the prior art is that most of the paints used today are based on organic solvents, particularly on aromatic hydrocarbon containing solvents. However, these solvents are sources of air pollution and to reduce their use drastically is a global concern.